Improvement of spot quality in femtosecond laser point-by-point inscription of fiber gratings

The influence of the intrinsic fiber geometry on the laser beam focusing during the point-bypoint inscription of fiber Bragg grating (FBG) was highlighted. The laser induced spot shows a strip profile because of the fiber cylindrical lens effect. The reason for this cylindrical lens effect was analyzed based on the principle of geometrical optics, and an effective method was used to improve spot quality. During the experiment, a fiber was immersed into the refractive index match oil whose index is 1.47, then a period structure with single pulse induced was inscribed into the core of this LMA double-cladding fiber by 50x microscope objective focused femtosecond laser. At last, the cylindrical lens effect was eliminated and the circle spot with the diameter of 1.5 μm was realized. There should be a very wide range of application in fiber microstructure fabricating.